Early cuticle formation in an adenophorean nematode

Abstract

The ultrastructure of cuticular formation was studied during the first 8 days of development in Romanomermis culicivorax. On day 1 the plasma membranes of the blastomeres show no specialization. On day 2 short pseudopodial extensions from some of the outer cells begin to migrate over adjacent cells. Small spherical amembraneous structures, which may be connected by fine strands, are present external to the outer layer of cells. During day 3 many of the surface cells are polarized as plaques and minute projections occur on the apical plasma membranes. In addition these epidermal cells now have long pseudopodia which cover adjacent cells and interconnect by numerous desmosomes. On day 4 the apical epidermal membrane forms regularly aligned plicae which contain plaque material and closely associated ribosomes. Externally, there are two delicate membrane-like layers: only the inner one follows the contours of the plicae. On day 5 a distinct granular cuticle 1 (C-1) is present. In addition, the second cuticle (C-2) contains a cortical zone with radial lamellae. Distinct annulae now circumscribe the worm. The narrow C-1 is firmly attached to a dark band (the future epicuticle) of C-2. A ‘foamy’ region lies between the epidermis and the cuticle and possibly represents an area of recently synthesized cuticular material. The epidermis contains projections from which cuticular material appears by exocytosis. At this time the cell body of the epidermal cell is internal. On day 6, a narrow osmiophilic clear band separates C-1 from C-2. C-2 has a trilaminate epicuticle and a median zone appears. On day 7 regions of C-1 are detached and the basal zone of C-2 is present with several layers. By day 8 C-1 has molted and C-2 consists of an epicuticle, cortical zone, a median zone with two or more indistinct layers, a basal zone consisting of five alternating dark and pale wavy fibrillar bands. A wide osmiophilic pale zone separates C-2 from the underlying epidermis.